Technical Field
The invention relates to a projection lens for imaging a pattern arranged in an object plane of the projection lens into an image plane of the projection lens via electromagnetic radiation having an operating wavelength λ<260 nm, and to a projection exposure method which can be carried out with the aid of the projection lens.
Prior Art
Nowadays, predominantly microlithographic projection exposure methods are used for producing semiconductor components and other finely structured components, such as e.g. photolithography masks. This involves using masks (reticles) or other patterning devices which bear or form the pattern of a structure to be imaged, e.g. a line pattern of a layer of a semiconductor component. The pattern is positioned in a projection exposure apparatus between an illumination system and a projection lens in the region of an object plane of the projection lens and is illuminated with an illumination radiation provided by the illumination system. The radiation altered by the pattern passes as prediction radiation through the projection lens, which images the pattern on a reduced scale onto the substrate to be exposed. The surface of the substrate is arranged in the image plane of the projection lens, the image plane being optically conjugated with respect to the object plane. The substrate is generally coated with a radiation-sensitive layer (resist, photoresist).
One of the aims in the development of projection exposure apparatuses is to lithographically produce structures having increasingly smaller dimensions on the substrate. Smaller structures lead to higher integration densities e.g. in semiconductor components, which generally has an advantageous effect on the performance of the microstructured components produced.
The size of the structures that can be produced is crucially dependent on the resolution capability of the projection lens used and can be increased on the one hand by reducing the wavelength of the projection radiation used for the projection, and on the other hand by increasing the image-side numerical aperture NA of the projection lens that is used in the process.
High-resolution projection lenses nowadays operate at wavelengths of less than 260 nm in the deep ultraviolet range (DUV) or in the extreme ultraviolet range (EUV).
In order, at wavelengths from the deep ultraviolet range (DUV), to ensure sufficient correction of aberrations (e.g. chromatic aberrations, image field curvature), catadioptric projection lenses are usually used, which contain both transparent refractive optical elements having refractive power (lens elements) and reflective elements having refractive power, that is to say curved mirrors. Typically, at least one concave mirror is contained. Resolution capabilities that enable projection of structures having a size of 40 nm are achieved nowadays with immersion lithography at NA=1.35 and λ=193 nm.
Integrated circuits are produced by a sequence of photolithographic patterning steps (exposures) and subsequent process steps, such as etching and doping, on the substrate. The individual exposures are usually carried out with different masks or different patterns. In order that the finished circuit exhibits the desired function, it is necessary for the individual photolithographic exposure steps to be coordinated with one another as well as possible, with the result that the fabricated structures, for example contacts, lines and the constituents of diodes, transistors and other electrically functional units, come as close as possible to the ideal of the planned circuit layouts.
Fabrication faults can arise, inter alia, if the structures produced in successive exposure steps do not lie one on top of another sufficiently accurately, that is to say if the superimposition accuracy is not sufficient. The superimposition accuracy of structures from different fabrication steps of a photolithographic process is usually designated by the term “overlay”. This term denotes e.g. the superimposition accuracy of two successive lithographic planes. Overlay is an important parameter in the fabrication of integrated circuits since alignment errors of any type can cause fabrication faults such as short circuits or missing connections and thus restrict the functioning of the circuit.
In multiple-exposure methods, too, stringent requirements are made of the superimposition accuracy of successive exposures. In the double-patterning method (or double-exposure method), for example, a substrate, for example a semiconductor wafer, is exposed twice in succession and the photoresist is then processed further. In a first exposure process, e.g. a normal structure having a suitable structure width is projected. For a second exposure process, a second mask is used, having a different mask structure. By way of example, periodic structures of the second mask can be displaced by half a period relative to periodic structures of the first mask. In the general case, particularly for more complex structures, the differences between the layouts of the two masks can be large. Double patterning makes it possible to achieve a reduction of the period of periodic structures on the substrate. This can be accomplished only if the superimposition accuracy of the successive exposures is good enough, that is to say if the overlay errors do not exceed a critical value.
Inadequate overlay can thus considerably reduce the yield of good parts during fabrication, as a result of which the fabrication costs per good part increase.